Recently, intensive studies have been made about semiconductor memory elements using a highly dielectric thin film of a high dielectric constant compared with a silicon oxide film, and about those using a ferroelectric thin film with spontaneous polarization. Highly dielectric materials typically include oxides such as Strontium Titanate (STO, SrTiO.sub.3) and Barium/Strontium Titanate (BSTO, (Ba.cndot.Sr)TiO.sub.3) Ferroelectric materials typically include oxides such as Lead Zirconate Titanate (PZT, Pb(Zr.cndot.Ti)O.sub.3), Lead Titanate (PbTiO.sub.3), Barium Titanate (BaTiO.sub.3), Lead Lanthanum Zirconate Titanate (PLZT, (Pb.cndot.La) (Zr.cndot.Ti)O.sub.3), and Bismuth Layered Oxides (BTO, Bi.sub.4 Ti.sub.3 O.sub.12 ; SrBi.sub.2 (Ta.sub.x Nb.sub.1-x).sub.2 O.sub.9) Especially, the mainstream study now is concerned with PZT and Bismuth Layered Oxide which seem to be the most promising materials for non-volatile memories.
FIG. 7 is a cross-sectional view showing a structure of a first conventional semiconductor memory element. The semiconductor memory element using a conventional highly dielectric material or ferroelectric material for a dielectric film of a capacitor employs, as illustrated in FIG. 7, a stack type structure in which a capacitor composed of a lower electrode 27, a dielectric film 28 and an upper electrode 30 is formed on a selective transistor composed of a gate electrode 23 and source/drain areas 24. The structure reduces a memory cell area, thereby facilitating high integration. In order to realize such a stack type structure, wiring 26 connecting the selective transistor and the capacitor needs to have a plug structure. In FIG. 7, 21 is a semiconductor substrate (e.g., an n type silicon substrate); 22 is a LOCOS (Local Oxidation of Silicon) oxide film for separating elements; 25, 29 and 31 are interlayer insulating films; and 32 and 33 are wiring electrodes.
At present, polysilicon and tungsten are widely used as a material for a plug of a minute contact hole. Besides, platinum is used as a material for the lower electrode of the capacitor because of its resistance to oxidation and reaction.
In a process of forming the highly dielectric film or ferroelectric film used in the capacitor, a treatment in oxidizing atmosphere at a high temperature between 500.degree. and 700.degree. C. is essential to obtain the great dielectric constant or ferroelectricity by crystallization of these films. The highly integrated semiconductor memory element has problems, such as deteriorating transistor properties, in its application to practical use. Such deterioration is caused by (1) reaction between the platinum lower electrode of the capacitor and either the polysilicon plug or the tungsten plug at high temperatures during the dielectric film forming process, (2) poor contact due to oxidation of the plug, or (3) diffusion of lead, Bi, etc. in the platinum, the ferroelectric film, etc. These problems creates needs for a thermally stable conductive diffusion barrier layer having a strong barrier capability to oxygen, platinum, lead, Bi and the like to be disposed between the plug and the platinum lower electrode.
For example, in Official Gazette of United States Patent and Trademark Office U.S. Pat. No. 5,005,102, a titanium layer 34 and a titanium nitride layer 35 are used as the diffusion barrier layer as shown in FIG. 8. Nevertheless, if oxygen annealing is conducted at 600.degree. C. in order to crystallize the dielectric film 28, the diffusion barrier layer is oxidized, thereby causing a stress change in the diffusion barrier layer. iliere occurs peel-off at an interface between the lower electrode and a TiN film to relieve this stress change. More specifically, the stress change results from a change of compressive stress due to volume expansion caused by oxidation of the TiN film during crystallization of a PZT film. For example, the TiN film has a tensile stress of about 5.times.10.sup.9 dyne/cm.sup.2 at room temperature.
Moreover, oxygen, lead, Bi, platinum, etc. diffuse through a grain boundary with ease because the titanium layer 34 and the titanium nitride layer 35 used as the diffusion barrier layer takes a column ar crystal structure. Therefor, the film thickness needs to be not less than 2000 .ANG. to obtain enough barrier capability. This causes a greater stage difference in a capacitor area and hinders high integration. These are very serious problems in the application of the highly integrated semiconductor memory element into practical use.
Japanese Laid-Open Patent Application No. 4-85878/1992 (Tokukaihei 4-85878) discloses a configuration of using an ITO film as a lower electrode. Nonetheless, properties of the ITO film deteriorate at temperatures higher than 500.degree. C. Especially at 600.degree. C. where the ferroelectricity sinters, the ITO film has a problem of an increasing resistance.